The dies for use in molding resins (e.g., expansion molding, injection molding, compression molding and blow molding) or in casting metals (e.g., low-pressure casting and die casting of aluminum alloys, etc.) must be formed with vent ports for the escape of air from inside the cavity or a gas evolved from the resin or cast metal. Heretofore used as such dies are those wherein a vent port is formed in the required portion by machining.
To improve the efficiency of molding or casting operation and the quality of moldings or castings, attempts have recently been made to use dies having a porous metal body locally incorporated therein or those entirely made of a porous metal body. Various proposals have been made as to processes for producing porous bodies and construction of dies (JP-A-1-205846, JP-A-3-170656, JP-A-4-72004, JP-A-4-83603, JP-A-4-339624, etc.).
Porous metal bodies are useful also for a wide variety of applications. Besides the application to dies and attachments therefor, it is attempted to use such bodies as filters for gases and fine particles, as sensors and catalyst carriers exploiting their feature of having a great specific surface area, as soundproof or damping materials utilizing vibration transmission attenuating characteristics, and as heat-insulating materials utilizing the heat-insulating properties of the porous structure.
To be useful for these applications, the porous metal body needs to have its porosity and pore sizes suitably adjusted in conformity with the mode of use and to possess the required mechanical properties. Various processes have been proposed for producing such porous metal bodies. These proposals include a process wherein a mixture of metal fiber as of stainless steel adjusted to a specified size and metal powder is pressed to shape, and the shaped powder body is sintered in a vacuum or reducing atmosphere, a process where such a shaped power body as sintered is subjected to a nitriding treatment to give an increased strength and higher hardness, and a process wherein a powder starting material is pressed to shape at a low pressure and further pressed at a higher pressure to obtain a body of powder as shaped by two-step pressing, and the shaped powder body is thereafter sintered in a specified atmosphere. (See, for example, JP-A-3-239509, JP-A4-72004, JP-A-4-308048, and JP-A-6-33112)
With these conventional processes, the shaped powder body obtained by pressing a powdery material is sintered at atmospheric pressure, and this presents difficulty in controlling the porosity and pore size of the porous metal body to be prepared, and is liable to make the body uneven in porosity and pore size. The drawback becomes more pronounced as the product increases in size or becomes more complex in shape. Further with metal materials which are difficult to shape or sinter, it is difficult to assure the product of mechanical strength.
The mechanical strength of sintered bodies can be enhanced by conducting the sintering treatment at a higher temperature to promote the sintering reaction, whereas particles then fuse to one another to result in smaller pore sizes and a decreased porosity. The fusion of particles impairs the function of the sintered body as a porous material since the open porosity (proportion of the pores communicating with outside the surface to the pores of the body) of the sintered body then lowers.
For example in the case of the dies for injection molding or die casting conducted under a high pressure, it is desired to reduce the pore size and lower the porosity to assure the porous body of strength and rigidity, but the conventional process has the problem that small pores become closed to lower the open porosity, rendering the porous body unserviceable as such.
Further in the case of filters for use at a high temperature inside refuse incinerators for filtering a gas or fine particles, some kinds of filters need to have increased pore sizes to give a higher porosity. However, the conventional production process fails to achieve a sufficient bond strength between the particles and encounters difficulty in ensuring the desired strength and rigidity if the pore sizes are increased to increase the porosity.
The present invention provides a process for producing a porous metal body free of the foregoing drawbacks or problems, and the porous metal body.